Process for producing propylene copolymers

ABSTRACT

A process for producing a propylene polymer having good flowability with an MFR of at least 10 g/10 minutes comprises a first polymerization stage consisting of at least two steps and a subsequent second polymerization stage. It is important to control the intrinsic viscositites of a crystalline propylene polymer obtained from the first step of the first polymerization stage and a crystalline propylene polymer obtained from the final step of the first polymerization stage.

This application is a continuation of application Ser. No. 07/188,394filed Mar. 17, 1988, now abandoned.

TECHNOLOGICAL FIELD

This invention relates to a process for producing a propylene copolymer.More specifically, it relates to a process for producing a propylenecopolymer having excellent melt-flowability, moldability, rigidity,impact strength, powder flowability and low-temperature heat-sealabilityunder a low partial pressure of hydrogen with a high catalyst efficiencyand good operability.

BACKGROUND TECHNOLOGY

It has been known that in the presence of a stereospecific catalyst ofvarious types, a crystalline polymer or copolymer of propylene (the twowill sometimes be generically referred to as polypropylene) is producedin a first step, and in a second and a subsequent step, propylene andanother alpha-olefin are copolymerized in the presence of the abovepolypropylene to produce a crystalline polymer or copolymer of propyleneand/or the other alpha-olefin, above all a crystalline polymer orcopolymer of ethylene or a crystalline polymer or copolymer containingethylene as a main component. It is known that such a multiplicity ofsteps give a composition having improved impact strength at lowtemperatures while retaining the excellent rigidity of polypropylene.

The composition is usually a uniform and intimate mixture of thepolymers or copolymers produced in the individual steps, but isgenerally referred to as a block copolymer. The block copolymer gainswidespread use in containers, automobile parts, easily low-temperaturesealable films, high impact films, etc.

Japanese Laid-Open Patent Publication No. 115296/75 and itscorresponding U.S. Pat. No. 3,974,236 disclose a process for producing aheteroblock copolymer of propylene and ethylene in two stages describedbelow. The first stage consists of at least one step of producingisotactic polypropylene by polymerizing propylene in the presence of acatalyst and at least one step of producing an ethylene/propylenecopolymer by polymerizing a mixture of ethylene and propylene in thepresence of a catalyst. The ethylene content of this mixture is 0.2 to3% by weight, and the content of isotactic polypropylene is at most 25%by weight of the heteroblock copolymer produced in the first stage. Thefirst stage is started by polymerizing propylene and the two steps aresuccessively carried out. In the second stage, another copolymer ofethylene and propylene is produced by polymerizing a mixture of ethyleneand propylene in the presence of a catalyst until the quantity of thecopolymer reaches 5 to 20% by weight of the heteroblock copolymer to befinally produced. The ethylene content of the mixture is at least 50%.The above patent specifications state that the heteroblock copolymer hasexcellent surface gloss, high flexural rigidity, and good impactstrength.

British Patent No. 1,543,096 discloses a process for producing achemically blended propylene polymer composition suitable for use inproducing molded articles having excellent properties such as highimpact strength and rigidity.

This process comprises (i) producing crystalline polypropylenecomponent, (I) in a first step wherein propylene optionally containingup to 1 mole% of another olefin is polymerized in the presence of acatalyst composed of (A) a carrier-supported titanium catalyst componentcontaining at least magnesium, halogen and titanium on the surface ofthe carrier and (B) an organoaluminum compound, (ii) producing in asecond step wherein propylene and ethylene are copolymerized in thepresence of the reaction product of the first step and the same catalystwhile maintaining the content of propylene in the gaseous phase of thepolymerization zone at 65 to 90 mole%; and (iii) producing polyethyleneor an ethylene/propylene copolymer component (III) in a third stepwherein ethylene or both ethylene and propylene are polymerized in thepresence of the reaction product of the second step and the samecatalyst while maintaining the content of propylene in the gaseous phaseof the polymerization zone at 0 to 15 mole%.

British Patent No. 1,566,391 discloses a process for producing achemically blended propylene polymer composition having impactresistance and being suitable for production of molded articles havingexcellent properties such as high impact strength and rigidity andimproved whitening resistance and gloss.

U.S. Pat. No. 4,547,552 discloses a process for producing a propyleneblock copolymer composition suitable for producing molded articleshaving excellent impact strength, especially at low temperatures, andexcellent rigidity in a well balanced combination. This processcomprises (I) a first stage of polymerizing propylene containing 0 to 5mole% of another olefin in the presence of a catalyst composed of (A) asolid titanium catalyst component consisting essentially of titanium,magnesium, halogen, and an electron donor, (B) an organoaluminumcompound, and (C) an organic silicon compound having an Si-0-C bond oran Si-N-C bond to thereby form a crystalline propylene polymer orcopolymer; and (II) a second stage of polymerizing propylene andethylene, or propylene, ethylene and another olefin in the presence ofthe reaction product of the first stage and the same catalyst as used inthe first stage to form a rubbery propylene copolymer and a crystallineethylene polymer or copolymer.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a process for producing apropylene copolymer.

Another object of this invention is to provide a process for producing apropylene copolymer having excellent melt-flowability and moldabilityunder a low partial pressure of hydrogen with a high catalyticefficiency.

Still another object of this invention is to provide a process forproducing a propylene copolymer having excellent rigidity, impactstrength and powder flowability with a high catalytic efficiency.

Other objects of this invention along with its advantages will becomeapparent from the following description.

According to this invention, these objects and advantages of theinvention are achieved by a process for producing a propylene blockcopolymer having an MFR value of at least 10 g/10 minutes in thepresence of a catalyst system formed from

(A) a solid titanium catalyst component containing magnesium, titanium,halogen and a polycarboxylic acid ester as essential ingredients formedby contacting a magnesium compound, a titanium compound and thepolycarboxylic acid ester,

(B) an organoaluminum compound, and

(C) an organic silicon compound represented by the following formula (I)

    SiR.sup.11.sub.m (OR.sup.12).sub.4-m                       (I)

wherein R¹¹ represents an alkyl or alkenyl group, R¹² represents ahydrocarbon group, and m is 1≦m≦2,

which comprises

(I) polymerizing or copolymerizing propylene in a first polymerizationstage consisting of at least two steps wherein a crystalline propylenepolymer having an intrinsic viscosity [η]_(FC), measured in decalin at135° C., of less than 1.8 dl/g is produced in the first step, thepolymerization or copolymerization is carried out further in thesubsequent steps, and from the final step, a crystalline propylenepolymer having an intrinsic viscosity [η]_(XC), measured in decalin at135° C., of 0.7 to 2.0 dl/g is taken out, the [η]_(FC) /[η]_(XC) ratiobeing adjusted to not more than 0.9, and

(II) random-copolymerizing propylene and another alpha-olefin in a moleratio of from 10/90 to 90/10 in a second polymerization stage in thepresence of said crystalline propylene polymer.

The catalyst component (A) used in this invention is a highly activecatalyst component containing magnesium, titanium, halogen and thepolycarboxylic acid ester as essential ingredients. The titaniumcatalyst component (A) contains a magnesium halide having a smallercrystallite size than commercially available magnesium halides andusually has a specific surface area of at least about 50 m² /g,preferably about 60 to about 1000 m² /g, more preferably about 100 toabout 800 m² /g, and its composition does not substantially change whenwashed with hexane at room temperature. When a diluent, for example aninorganic or organic compound such as a silicon compound, an aluminumcompound or a polyolefin, is used, the titanium catalyst component (A)exhibits higher performance even when its specific surface is lower thanthat mentioned above. Preferably, in the titanium catalyst component(A), the halogen/titanium atomic ratio is from about 5 to about 100,especially from about 5 to about 100; the mole ratio of the electrondonor/titanium is from about 0.1 to about 10, especially from about 0.2to about 6; and the magnesium/titanium atomic ratio is from about 1 toabout 100, especially from about 2 to about 50. The component (A) maycontain another electron donor, metals, elements, functional groups,etc.

The titanium catalyst component (A) can be prepared, for example, bycontacting a magnesium compound (or metallic magnesium), an electrondonor and a titanium compound with each other, optionally in thepresence of another reaction agent such as a compound of silicon,phosphorus or aluminum.

Examples of the method of producing the titanium catalyst component (A)are disclosed, for example, in the specifications of Japanese Laid-OpenPatent Publications Nos. 108385/1975, 126590/1975, 20297/1976,28189/1976, 64586/1976, 92885/1976, 136625/1976, 87489/1977,100596/1977, 100596/1977, 147688/1977, 104593/1977, 2580/1977,40093/1978, 43094/1978, 135102/1980, 135103/1981, 811/1981, 11908/1981,18606/1981, 83006/1983, 138705/1983, 138706/1983, 138707/1983,138708/1983, 138709/1983, 138710/1983, 138715/1983, 23404/1985,21109/1986, 37802/1986, 37803/1986 and 152710/1980.

Some preferred methods for producing the titanium catalyst component (A)among them are described below.

(1) A magnesium compound or a complex of a magnesium compound is reactedwith a titanium compound which forms a liquid phase under the reactionconditions. Prior to the reaction, the magnesium compound or themagnesium complex may, or may not, be pulverized in the presence orabsence of an electron donor, a pulverization aid, etc., and may or maynot be pre-treated with an electron donor and/or an organoaluminumcompound, or a reaction aid such as a halogen-containing siliconcompound. In the above method, the electron donor is used at least once.

(2) A liquid form of a magnesium compound having no reducing ability isreacted with a liquid titanium compound in the presence of an electrondonor to precipitate a solid titanium complex.

(3) The product of (2) is reacted with a titanium compound.

(4) The product of (1) or (2) is reacted with an electron donor and atitanium compound.

(5) A magnesium compound or a complex of a magnesium compound and anelectron donor is pulverized in the presence of a titanium compound withor without an electron donor, a pulverization aid, etc., and theresulting solid is treated with halogen, a halogen compound or anaromatic hydrocarbon. In the above process, the pulverized product maybe pre-treated as required with an electron donor and/or anorganoaluminum compound or a reaction aid such as a halogen-containingsilicon compound. The electron donor is used at least once in the aboveprocess.

(6) The product of (1), (2), (3) or (4) is treated with halogen, ahalogen compound or an aromatic hydrocarbon.

(7) A reaction product obtained by contacting a metal oxide,dihydrocarbyl magnesium and a halogen-containing alcohol is contactedwith a polycarboxylic acid ester and a titanium compound.

(8) A magnesium salt of an organic acid, a magnesium compound such as analkoxymagnesium or aryloxymagnesium are reacted with a polycarboxylicacid ester, a titanium compound, and/or a halogen-containinghydrocarbon.

Especially preferred are those in which a liquid titanium halide isused, or a halogenated hydrocarbon is used during or after using atitanium compound.

The polycarboxylic acid ester is an electron donor which can be aningredient constituting the highly active titanium catalyst component(A) in this invention. Suitable polycarboxylic acid esters are thosehaving skeletons of the following formula ##STR1##

In these formulae, R¹ represents a substituted or unsubstitutedhydrocarbon group; R² represents hydrogen or a substituted orunsubstituted hydrocarbon group; R³ and R⁴ represent hydrogen or asubstituted or unsubstituted hydrocarbon group, preferably at least oneof them is a substituted or unsubstituted hydrocarbon group; and R³ andR⁴ may be linked to each other. The substituents may be those containinga hetero atom such as N, O or S, for example the C--O--C, COOR, COOH,OH, SO₃ H, --C--N--C and NH₂ groups.

Especially preferred are dicarboxylic acid diesters in which at leastone of R¹ and R² is an alkyl group having at least 2 carbon atoms.

Specific examples of preferred polycarboxylic acid esters includealiphatic polycarboxylic acid esters such as diethyl succinate, dibutylsuccinate, diethyl methylsuccinate, diisobutyl alpha-methylglutarate,dibutylmethyl malonate, diethyl malonate, diethyl ethylmalonate, diethylisopropylmalonate, diethyl butylmalonate, diethyl phenylmalonate,diethyl diethylmalonate, diethyl allylmalonate, diethyldiisobutylmalonate, diethyl di-n-butylmalonate, dimethyl maleate,monoctyl maleate, diisooctyl maleate, diisobutyl maleate, diisobutylbutylmaleate, diethyl butylmaleate, diisopropyl beta-methylglutarate,diallyl ethylsuccinate, di-2-ethylhexyl fumarate, diethyl itaconate,diisobutyl itaconate, diisooctyl citraconate and dimethyl citraconate;aliphatic polycarboxylic acid esters such as diethyl1,2-cyclohexanecarboxylate, diisobutyl 1,2-cyclohexanecarboxylate,diethyl tetrahydrophthalate and diethyl ester of Nadic acid; aromaticpolycarboxylic acid esters such as monoethyl phthalate, dimethylphthalate, methylethyl phthalate, monoisobutyl phthalate, diethylphthalate, ethylisobutyl phthalate, mono-n-butyl phthalate,ethyl-n-butyl phthalate, di-n-propyl phthalate, di-isopropyl phthalate,di-n-butyl phthalate, di-isobutyl phthalate, di-n-heptyl phthalate,di-2-ethylhexyl phthalate, didecyl phthalate, benzylbutyl phthalate,diphenyl phthalate, diethyl naphthalenedicarboxylate, dibutylnaphthalenedicarboxylate, triethyl trimellitate and dibutyltrimellitate; and esters of heterocyclic polycarboxylic acids such as3,4-furanedicarboxylic acid.

Other examples of the polycarboxylic acid ester that can be supported onthe titanium catalyst component include long-chain dicarboxylic acidesters such as ethyl adipate, diisobutyl adipate, diisopropyl sebacate,di-n-butyl sebacate, n-octyl sebacate and di-2-ethylhexyl sebacate.

Preferred polycarboxylic acid esters are those having skeletons of theabove-given general formulae. More preferred are esters formed betweenphthalic acid, maleic acid or a substituted malonic acid and alcoholshaving at least 2 carbon atoms. Diesters formed between phthalic acidand alcohols having at least 2 carbon atoms are especially preferred.

In supporting the above electron donor, the electron donor need notalways to be used as a starting material. It is possible to use acompound capable of being changed into an electron donor in the courseof preparing the titanium catalyst component, and convert it into theelectron donor during the preparation.

Another electron donor may be present in the titanium catalystcomponent. If it is present in too large an amount, adverse effects areexerted. Hence, its amount should be limited to a small value.

The magnesium compound used in the preparation of the solid titaniumcatalyst component (A) in this invention is a magnesium compound with orwithout reducing ability. Examples of the former are magnesium compoundshaving a magnesium-carbon bond or a magnesiumhydrogen bond, such asdimethylmagnesium, diethylmagnesium, dipropylmagnesium,dibutylmagnesium, diamylmagnesium, dihexylmagnesium, didecylmagnesium,ethyl magnesium chloride, propyl magnesium chloride, butyl magnesiumchloride, hexyl magnesium chloride, amyl magnesium chloride,butylethoxymagnesium, ethylbutylmagnesium and butylmagnesium hydride.These magnesium compounds may be used in the form of complexes withorganoaluminum, for example, and may be in the liquid or solid state.Examples of the magnesium compound without reducing ability aremagnesium halides such as magnesium chloride, magnesium bromide,magnesium iodide and magnesium fluoride; alkoxy magnesium halides suchas ethoxy magnesium chloride, isopropoxy magnesium chloride, butoxymagnesium chloride and octoxy magnesium chloride; aryloxy magnesiumhalides such as phenoxy magnesium chloride and methylphenoxy magnesiumchloride; alkoxymagnesiums such as ethoxymagnesium, isopropoxymagnesium,butoxymagnesium, n-octoxymagnesium and 2-ethylhexoxymagnesium;aryloxymagnesiums such as phenoxymagnesium and dimethylphenoxymagnesium;and carboxylic acid salts of magnesium such as magnesium laurate andmagnesium stearate. The magnesium compound having no reducing abilitymay be derived from the magnesium compound having reducing ability ormay be derived during preparation of the catalyst component. Forexample, by contacting the magnesium compound having reducing abilitywith a polysiloxane compound, a halogen-containing silane compound, ahalogen-containing aluminum compound, an ester, and an alcohol, it canbe converted into a magnesium compound having no reducing ability. Theabove magnesium compound may be a complex or binary compound withanother metal, or a mixture with another metal compound. Of these,magnesium compounds having no reducing ability are preferred.Halogen-containing magnesium compounds, above all magnesium chloride,alkoxy magnesium chlorides and aryloxy magnesium chlorides, arepreferred.

In the present invention, tetravalent titanium compounds of thefollowing formula

    Ti(OR).sub.g X.sub.4-g

wherein R represents a hydrocarbon group, X represents halogen, and g isa number of 0 to 4,

are preferred as examples of the titanium compound used in preparing thesolid titanium catalyst component (A). Examples of such compoundsinclude titanium tetrahalides such as TiC₁₄, TiBr₄ and TiI₄ ; alkoxytitanium trihalides such as Ti(OCH₃)Cl₃, Ti(OC₂ H₅)Cl₃, Ti(O n-C₄H₉)Cl₃, Ti(OC₂ H₅)Br₃ and Ti(O iso-C₄ H₉)Br₃ ; alkoxy titanium dihalidessuch as Ti(OCH₃)₂ Cl₂, Ti(OC₂ H₅)₂ Cl₂, Ti(O n-C₄ H₉)₂ Cl₂ and Ti(OC₂H₅)Br₂ ; trialkoxy titanium monohalides such as Ti(OCH₃)₃ Cl , Ti(OC₂H₅)₃ Cl, Ti(O n-C₄ H₉)₃ Cl and Ti(OC₂ H₅)₃ Br; and tetraalkoxy titaniumTi(OCH₃)₄, Ti(OC₂ H₅)₄ and Ti(O n-C₄ H₉)₄. Of these, thehalogen-containing titanium compounds, especially titanium tetrahalides,are preferred. Titanium tetrachloride is especially preferred. Thesetitanium compounds may be used singly or as a mixture, or as dilutedwith a hydrocarbon or a halogenated hydrocarbon.

The amounts of the titanium compound, the magnesium compound and theelectron donor to be deposited, and the electron donor which may be usedas required (such as an alcohol, phenol, and monocarboxylic acid esters,the silicon compound, the aluminum compound, etc.) in the preparation ofthe titanium catalyst component (A) differ depending upon the method ofpreparation, and cannot be generalized. For example, about 0.01 to 5moles of the electron donor to be deposited and about 0.01 to 500 molesof the titanium compound may be used per mole of the magnesium compound.

In the present invention, an olefin is polymerized or copolymerizedusing a catalyst composed of the titanium catalyst component (A), anorganoaluminum compound catalyst component (B) and an organic siliconcompound (C).

Examples of (B) may include:

(i) organoaluminum compounds containing at least one Al-C bond in themolecule, for example organoaluminum compounds of the following formula

    R.sub.l.sup.7 Al(OR.sup.8).sub.i H.sub.p X.sub.q

wherein R⁷ and R⁸ represent a hydrocarbon group usually having 1 to 15carbon atoms and may be identical or different, X represents halogen,0≦l≦3, 0≦i≦3, 0≦p≦3, and 0≦q≦3, and l+i+p+q=3;

(ii) complex alkylated products formed from metals of Group I andaluminum, which are represented by the following formula

    M.sup.1 AIR.sup.7.sub.4

wherein M¹ represents Li, Na and K, and R⁷ is as defined above; and

(iii) organoaluminum compounds in which two or more aluminums are bondedvia an oxygen or nitrogen atom.

Examples of the organoaluminum compound belonging to (i) include:

compounds represented by the following formula

    R.sub.1.sup.7 Al(OR.sup.8).sub.3-l

wherein R⁷ and R⁸ are as defined above and l is preferably 1.5≦l≦3;

compounds of the following formula

    R.sub.l.sup.7 AlX.sub.3-l

wherein R⁷ is as defined, X represents halogen, and l is preferablyO<l<3;

compounds of the following formula

    R.sub.l.sup.7 AlH.sub.3-l

wherein R⁷ is as defined, and l is preferably 2≦l≦3; and compounds ofthe following formula

    R.sub.l.sup.7 Al(OR.sup.8).sub.i X.sub.q

wherein R⁷ and R⁸ are as defined above, X is halogen, 0<l≦3, 0≦i<3, and0≦q<3, and l+i+q=3;

Specific examples of the aluminum compounds (i) include trialkylaluminums such as triethyl aluminum and tributyl aluminum; trialkenylaluminums such as triisoprenyl aluminum; dialkyl aluminum alkoxides suchas diethyl aluminum ethoxide and dibutyl aluminum butoxide; alkylaluminum sesquialkoxides such as ethyl aluminum sesquiethoxide and butylaluminum sesquibutoxide; partially alkoxylated alkyl aluminums having anaverage composition of the formula R₂.5⁷ Al(OR⁸)₀.5 ; dialkyl aluminumhalides such as diethyl aluminum chloride, dibutyl aluminum chloride anddiethyl aluminum bromide; alkyl aluminum sesquihalides such as ethylaluminum sesqauichloride, butyl aluminum sesquichloride and ethylaluminum sesquibromide; partially halogenated alkyl aluminums, forexample alkyl aluminum dihalides such as ethyl aluminum dichloride,propyl aluminum dichloride and butyl aluminum dibromide; dialkylaluminum hydrides such as diethyl aluminum hydride and dibutyl aluminumhydride; partially hydrogenated alkyl aluminums, for example alkylaluminum dihydride such as ethyl aluminum dihydride and propyl aluminumdihydride; and partially alkoxylated and halogenated alkyl aluminumssuch as ethyl aluminum ethoxy chloride, butyl aluminum butoxy chlorideand ethyl aluminum ethoxy bromide.

Examples of the compounds (ii) are LiAl(C₂ H₅)₄ and LiAl(C₇ H₁₅)₄.

Examples of the compounds (iii) are (C₂ H₅)₂ AlOAl(C₂ H₅)₂,(C₄ H₉)₂AlOAl(C₄ H₉)₂, ##STR2## and methyl aluminoxane.

Among these, trialkyl aluminums and alkyl aluminums (iii) in which twoor more aluminum are bonded are preferred.

The organic silicon compound (C) used in the invention is an organicsilicon compound represented by the following formula (I)

    SiR.sup.11.sub.m(OR.sup.12).sub.4-m                        (I)

wherein R¹¹ represents an alkyl or alkenyl

group, R¹² represents a hydrocarbon group and m is 1≦m≦2.

In formula (I), R¹¹ may be, for example, an alkyl group having 1 to 10carbon atoms such as a methyl, ethyl, propyl, butyl, pentyl, hexyl,octyl or decyl group, and an alkenyl group having 2 to 6 carbon atomssuch as a vinyl, allyl or propenyl group. R¹² may be, for example, analkyl, cycloalkyl, aryl or aralkyl group. Methyl and ethyl groups areparticularly preferred. Organic silicon compounds of formula (I) inwhich m is 1 are especially preferred. Examples of the organic siliconcompounds include trialkoxysilanes such as methyltriethoxysilane,ethyltriethoxysilane, vinyltriethoxysilane, n-propyltriethoxysilane,n-butyltriethoxysilane, n-hexyltriethoxysilane, n-octyltripropoxysilane,n-decyltrimethoxysilane and n-decyltriethoxysilane; and dialkoxysilanessuch as dimethyldimethoxysilane, diethyldimethoxysilane,di-n-propyldimethoxysilane, di-n-butyldiethoxysilane,di-n-hexyldiethoxysilane, di-n-octylpropoxysilane,di-n-decyldibutoxysilane, methylcyclohexyldimethoxysilane andethylcyclohexyldiethoxysilane.

According to the process of this invention, the propylene copolymer isproduced by polymerizing or copolymerizing propylene in a firstpolymerization stage consisting of at least two steps in the presence ofthe above-described catalyst to produce a crystalline propylene polymer,and subsequently random-copolymerizing propylene with anotheralpha-olefin in a mole ratio of from 10/90 to 90/10 in a secondpolymerization stage in the presence of the crystalline propylenepolymer. The first polymerization stage is carried out in polymerizationvessels in at least two steps, preferably at least three steps. Thesecond polymerization stage may be carried out in a singlepolymerization vessel or in two or more polymerization vessels. In theprocess of this invention, each step in the first and secondpolymerization stages may be carried out by vapor-phase polymerizationor liquid-phase polymerization. In the case of the liquidpolymerization, an inert hydrocarbon may be used as a liquid phase. Thestarting olefin may be used as a liquid medium.

Advantageously, the amount of the catalyst component (A) used is, forexample, about 0.005 to about 0.5 millimole, calculated as Ti atom, perliter of the polymerization volume.

The amount of the organoaluminum compound (B) is advantageously suchthat the proportion of the metal atom in the component (B) is about 1 toabout 2000 moles, preferably about 5 to about 500 moles, per mole of thetitanium atoms in the component (A) in the polymerization system.

The amount of the organic silicon compound (C) is advantageously about0.001 to about 10 moles, preferably about 0.01 to about 2 moles,especially preferably about 0.05 to about 1 mole, calculated as Si atomsin component (C) per mole of the metal atoms in the component (B).

The catalyst components (A), (B) and (C) may be contacted with oneanother during or before the polymerization. In the contacting beforepolymerization, any two of them alone may be contacted, or it ispossible to contact portions of two or three of them with one another.The contacting of the components before polymerization may be effectedin an atmosphere of an inert gas or in an atmosphere of an olefin.

The catalyst used in the pre-polymerization of the olefin before thefirst stage polymerization of propylene may be used as thepolymerization catalyst.

When this catalyst used in the pre-polymerization of the olefin is usedas the polymerization catalyst in the block copolymerization ofpropylene, it may be directly fed to the polymerization system, or afterit is washed with an inert hydrocarbon. The proportions of theindividual catalyst components are within the aforesaid ranges even whenthe catalyst used in the pre-polymerization is used as thepolymerization catalyst. When the individual components are used onlypartly in the prepolymerization treatment, the remainder of the catalystcomponents are fed in the first polymerization stage in the blockcopolymerization of propylene.

When propylene is polymerized or copolymerized using the catalyst usedin the pre-polymerization of the olefin, the polymerization activity andthe stereospecificity of the catalyst are further improved.Particularly, the resulting powdery polymer is spherical and hasexcellent uniformity and a high bulk density. In addition, in the caseof slurry polymerization, the properties of the slurry are excellent.Hence, the handlability of the powder or slurry is excellent.

In the pre-polymerization, about 0.1 to about 500 g, preferably 0.3 toabout 300 g, per gram of the component (A), of an olefin is preliminarypolymerized in the presence of at least part of the organoaluminumcompound (B). At this time, part or the whole of the organic siliconcompound (C) may be present in the prepolymerization system. The amountof the organoaluminum compound (B) may be one which is sufficient topolymerize the olefin in the above amount per gram of the component (A).Preferably, it is, for example, about 0.1 to about 100 moles, especiallyabout 0.5 to about 50 moles, per titanium atom in the highly activetitanium catalyst component (A).

Preferably, the pre-polymerization is carried out in an inerthydrocarbon medium or a liquid monomer used in the pre-polymerization,under mild conditions. The inert hydrocarbon medium used for thispurpose may be selected, for example, from the above-given examples ofthe inert media which can be used in halogenating organic magnesiumcompounds or their organic media. The prepolymerization treatment may becarried out batchwise or continuously. It may be carried out in a muchhigher catalyst concentration than the concentration of the catalyst inthe main polymerization system, and this is rather preferred. It is moreefficient therefore to carry it out batchwise.

The concentration of the highly active titanium catalyst component (A)in the pre-polymerization treatment is about 0.01 to about 200millimoles, preferably about 0.05 to about 100 millimoles, calculated astitanium atom, per liter of the inert hydrocarbon medium. Thetemperature used in the pre-polymerization treatment is one at which theresulting pre-polymer is substantially insoluble in the medium, and isusually about -20 to about +100° C., preferably about -20 to about +80°C., especially preferably from 0 to about +40° C. The above treatmentcan be carried out by feeding a predetermined amount of the olefin intoa suspension of the catalyst in an inert solvent. The olefin used forthis purpose may be, for example, ethylene, propylene, 1-butene,4-methyl-1-pentene or 1-octene. Those which produce highly crystallinepolymers are preferred. Propylene, 4-methyl-1pentene and 1-butene areespecially preferred. In the pre-polymerization, a molecular weightcontrolling agent such as hydrogen may be caused to be present. Theamount of the molecular weight controlling agent is preferably limitedto one in which a prepolymer having an intrinsic viscosity [η], measuredin decalin at 135° C., of at least 0.2 dl/g, preferably about 0.5 toabout 10 dl/g, can be produced.

The amount of the olefin pre-polymerized is about 0.1 to about 500 g,preferably about 0.3 to about 300 g, per gram of the titanium catalystcomponent (A). Since an increase in its amount does not correspondinglybring about an increase in effect, it is preferably limited to the aboverange.

The catalyst subjected to the pre-polymerization treatment is usedtogether with the organoaluminum compound (B) and the organic siliconcompound (C) (if the latter are not used in the pre-polymerizationtreatment), and block copolymerization of propylene is carried out.

In the process of this invention, the first polymerization stageconsists of at least two steps, preferably three steps. By polymerizingor copolymerizing propylene in this polymerization stage, a crystallinepropylene polymer is formed. In the first step of the firstpolymerization stage, the polymerization is carried out so that acrystalline polymer having an intrinsic viscosity [η]_(FC), measured indecalin at 135° C., of less than 1.8 dl/g is formed

If the crystalline propylene polymer formed in the first step has anintrinsic viscosity [η]_(FC) of more than 1.8 dl/g, the final blockcopolymer having an MFR of at least 10 undesirably has very weakdynamical strength. The [η]_(FC) of the crystalline propylene polymerformed in the first step is preferably 1.5 to 0.4 dl/g, especiallypreferably 1.0 to 0.45 dl/g.

The polymerization in the first stage is carried out such that from itsfinal step, a crystalline propylene polymer having an intrinsicviscosity [η]_(XC), measured in decalin at 135° C., of 0.7 to 2.0 dl/gis taken out.

If the intrinsic viscosity [η]_(XC) of the crystalline propylene polymerobtained from the final step is larger than 2.0 dl/g, the blockcopolymer having an MFR of at least 10 has inferior dynamical strength.If it is less than 0.7 dl/g, it is difficult to obtain block copolymerin an intimately mixed state.

Preferably, the [η]_(XC) of the crystalline propylene polymer obtainedfrom the final step is 1.0 to 1.5 dl/g.

The polymerization in the final step is carried out such that the[η]_(XC) of the crystalline propylene polymer obtained in the final stepsatisfies the above-specified range and the [η]_(FC) /[η]_(XC) ratio isnot more than 0.9. If the [η]_(FC) /[η]_(XC) ratio is larger than 0.9,it is disadvantageous to the production of a block copolymer in anintimately mixed state.

The [η]_(FC) /[η]_(XC) ratio is preferably not more than 0.8, especiallypreferably 0.78 to 0.5.

The proportion of the crystalline propylene polymer formed in the firststep of the first stage is usually 10 to 95% by weight, preferably 10 to90% by weight, most preferably 15 to 90% by weight, based on thecrystalline propylene polymer obtained from the last step of the firstpolymerization stage, and usually 2 to 90% by weight, preferably 5 to85% by weight, based on the propylene copolymer as a final product. Theproportion of the crystalline propylene polymer formed in the firststage polymerization step is usually 50 to 95% by weight, preferably 60to 90% by weight, based on the propylene copolymer finally produced. Thecrystalline propylene polymer obtained from the final step of the firstpolymerization stage has a stereospecificity index, measured by ¹³C-NMR, of at least 85%, especially at least 90%.

In the first polymerization stage, propylene is usually homopolymerized.Insofar as the aforesaid crystalline propylene polymer is formed, asmall amount (for example, not more than 10 mole%) of an alpha-olefinsuch as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octenemay be copolymerized with propylene.

The temperature at which propylene is polymerized or copolymerized inthe first polymerization stage may be properly changed so long as ahighly stereospecific highly crystalline polymer can be obtained.Preferably, it is about 20 to about 200° C., especially about 50 toabout 90° C. The polymerization pressure is, for example, fromatmospheric pressure to about 100 kg/cm² especially about 2 to about 50kg/cm². The polymerization conditions in each of the steps of the firstpolymerization stage are properly selected within the aforesaid ranges.

In the second polymerization stage of the present invention, propyleneand another alpha-olefin are random-copolymerized in a mole ratio offrom 10/90 to 90/10, preferably 20/80 to 80/20, in the presence of thecrystalline propylene polymer formed in the previous step and containingthe catalyst still having polymerization activity. The randomcopolymerization in the second polymerization stage may be carried outin the liquid phase or in the vapor phase. Since, however, a copolymermay form which dissolves in the liquid medium, it is preferred in viewof the yield of the final product to carry out the blockcopolymerization in the vapor phase. The temperature and pressure usedin the random copolymerization in the second polymerization stage may beproperly selected from the same ranges as shown for the production ofthe crystalline propylene polymer in the first polymerization stage. Theother alpha-olefin used as a comonomer in this stage may be for exampleethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene or 1-decene.It is preferably ethylene or a combination of ethylene with analpha-olefin having at least 4 carbon atoms.

In the production of the block copolymer in accordance with thisinvention, a step of producing a crystalline polymer or copolymer ofanother alpha-olefin may be provided so as to be carried outsubsequently to the above step of random copolymerization. Inparticular, the provision of a step of producing a crystalline polymeror copolymer of ethylene is effective for improving the impact strengthof the resulting copolymer.

The polymerization is preferably carried out such that the proportion ofthe random copolymer or a mixture of it with another crystallinealpha-olefin polymer is about 5% by weight to about 50% by weight,preferably about 1 to about 40% by weight of the copolymer compositionof this invention. When a step of producing a crystalline alpha-olefinpolymer is further provided, the polymerization is preferably carriedout such that the proportion of the crystalline alpha-olefin polymerbecomes 0 to about 50% by weight, preferably about 0 to about 35% byweight, based on the resulting copolymer.

According to this invention, a propylene copolymer having excellent meltflowability, moldability, rigidity, impact strength and powderflowability can be produced with a high catalytic efficiency and goodoperability.

EXAMPLES

The following examples further illustrate the present invention.

EXAMPLE 1 [Preparation of a solid Ti catalyst component (A)]

Anhydrous magnesium chloride (7.14 g; 75 mmoles), 37.5 ml of decane and35.1 ml (225 mmoles) of 2-ethylhexyl alcohol were reacted at 130° C. for2 hours to form a uniform solution. Phthalic anhydride (1.67 g; 11.3mmoles) was added to the solution, and the mixture was stirred at 130°C. for 1 hour to dissolve phthalic anhydride in the uniform solution.The uniform solution so obtained was cooled to room temperature and thenwholly added dropwise over 1 hour to 200 ml (1.8 moles) of titaniumtetrachloride kept at -20° C. After the addition, the mixed solution washeated to 110° C. over 4 hours, and when the temperature reached 110°C., 5.03 ml (18.3 mmoles) of diisobutyl phthalate was added. The mixturewas maintained at this temperature for 2 hours with stirring. After the2-hour reaction, the solid portion was collected by hot filtration. Thesolid portion was suspended in 275 ml of TiCl₄, and again reacted at110° C. for 2 hours. After the reaction, the solid portion was collectedby hot filtration and washed fully with decane and hexane at 110° C.until no free titanium compound was detected from the washings. Thesolid Ti catalyst component (A) prepared by the above method was storedas a hexane slurry. Part of it, however, was taken before storage, anddried in order to examine its composition. The solid Ti catalystcomponent (A) was found to contain 2.6% by weight of titanium, 58% byweight of chlorine, 18% by weight of magnesium and 12.4% by weight ofdiisobutyl phthalate.

[Pre-polymerization]

A 400 ml nitrogen-purged glass reactor was charged with 200 ml ofpurified hexane, and 6 mmoles of triethylaluminum and 2 mmoles,calculated as titanium atom, of the Ti catalyst component (A) wereintroduced. Then, propylene was fed into the reactor at a rate of 5.9liters/hour for one hour, and 2.8 g, per gram of the Ti catalystcomponent (A), of propylene was polymerized. The product was filtered toremove the liquid portion, and the separated solid portion wasreslurried in decane.

[Polymerization]

(1) A 50-liter autoclave was fully purged with propylene. Propylene(13.5 kg) and 90 Nl of hydrogen were introduced into the autoclave. Theautoclave was charged with 7.2 mmoles of triethylaluminum, 0.9 millimoleof n-propylenetriethoxysilane and 0.09 mg-atom, calculated as Ti atom,of the Ti catalyst component at 65° C., and propylene was polymerized at70° C. for 20 minutes.

(2) By releasing the pressure, propylene was removed, and thepolymerization was carried out for 1 hour at 80° C. while feedingpropylene so as to maintain the pressure of the inside of the autoclaveat 15 kg/cm² G.

(3) One hour later, the pressure was released, and a gaseous mixture ofethylene and propylene (ethylene/ propylene mole ratio 65/35) was fed ata rate of 375 Nl/hr at 60° C. for 7 hours to form a block copolymer. Nolumpy product was seen to form in the autoclave, nor was there anyadhesion to the wall of the autoclave. A polymer having good flowabilityand a high bulk density was obtained. The amount of the block copolymeryielded was 7.1 . The block copolymer had a melt flow index (measuredunder a load of 2.13 k9 at 230° C.) of 17.5 kg/10 minutes, an [η] of2.28 dl/g and an ethylene content of 14.9 mole% and contained 9.0% byweight of a component soluble in n-decane at room temperature.

An antioxidant was added to the block copolymer, and the mixture wasgranulated. Test pieces were prepared from the granules. The blockcopolymer had a falling dart impact strength (-20° C.) of 200 kg-cm, anIzod impact strength (0° C.) of 13.5 kg-cm/cm and a flexural modulus of14800 kg/cm².

The polypropylene obtained in (1) above (the first step of the firstpoIymerization stage) had an intrinsic viscosity [η]_(FC) of 1.0 dl/gand a boiling n-heptane extraction residue of 97%.

From the polymerization step (2) (the final step of the firstpolymerization stage), polypropylene having an intrinsic viscosity[η]_(XC) of 1.3 dl/g was obtained.

The [η]_(FC) /[η]_(XC) ratio was therefore 0.77. The amount ofpolymerization in the above first-stage polymerization steps (1) and (2)was 50% based on that in the propylene polymerization step.

COMPARATIVE EXAMPLE 1

(1) In polymerization (1) in Example 1, the amount of hydrogen waschanged from 90 Nl to 60 Nl, and the polymerization was carried out at70° C. for 30 minutes.

(2) Then, propylene was removed by releasing the pressure, and 40 Nl ofhydrogen was added at 80° C. While adding propylene so as to maintainthe remaining pressure at 15 kg/cm² G, the polymerization was carriedout for 40 minutes.

(3) After releasing the pressure, a gaseous mixture of ethylene andpropylene (ethyene/propylene mole ratio 65/35) was fed at a rate of 375Nl/hr for 100 minutes at 60° C. to form a block copolymer. The amount ofthe copolymer yielded was 6.8 kg, and it had an apparent density of 0.45g/ml, a melt flow index of 22 g/10 minutes, an [η] of 2.14 dl/g andethylene content of 14.1 mole%. It contained 8.8% by weight of acomponent soluble in decane at room temperature. An antioxidant wasadded to the block copolymer, and the mixture was granulated. Testpieces were prepared from the granules. The block copolymer was found tohave a falling dart impact strength (-20° C.) of 170 kg-cm, an Izodimpact strength (0° C.) of 14.8 kg-cm/cm and a flexural modulus of 14500kg/cm².

The ratio of the [η]_(FC) of the polypropylene formed in thepolymerization step (1) to the [η]_(XC) of the polypropylene formed inthe polymerization step (2), [η]_(FC) /[η]_(XC), was 1.0.

EXAMPLE 2 [Preparation of a solid catalyst component (A)]

A 2-liter high-speed agitating device (made by Tokushu Kika Kogyo) wasfully purged with nitrogen, and charged with 700 ml of purifiedkerosene, 10 g of commercial MgCl₂, 24.2 g of ethanol and 3 g of Emasol320 (a tradename for sorbitan distearate produced by Kao-Atlas Co.,Ltd.). The materials were heated with stirring, and stirred at 120° C.and 800 rpm for 30 minutes. With stirring at high speed, the mixture wastransferred to a 2-liter glass flask (equipped with a stirrer) filledwith 1 liter of purified kerosene cooled at -10° C. by using a Teflontube having an inside diameter of 5 mm. The resulting solid wascollected by filtration, and fully washed with hexane to obtain acarrier.

The carrier (7.5 g) was suspended at room temperature in 150 ml oftitanium tetrachloride, and 1.3 ml of diisobutyl phthalate was added.The mixture was heated to 120° C. The mixture was stirred at 120° C. for2 hours, and the solid portion was collected by filtration and suspendedin 150 ml of titanium tetrachloride, and the suspension was stirred at130° C. for 2 hours. The . solid was collected from the reaction mixtureby filtration and washed with a sufficient amount of purified hexane togive a solid titanium component (A). This catalyst component was foundto contain 2.3% by weight of titanium, 63% by weight of chlorine, 20% byweight of magnesium and 8.1% by weight of diisobutyl phthalate.

[Pre-polymerization]

Purified hexane (200 ml) was introduced into a 400 ml glass reactorpurged with nitrogen. Then, the reactor was charged with 6 mmoles oftriethylaluminum and 2 mmoles, as titanium atom, of the Ti catalystcomponent (A), and propylene was fed into the reactor at a rate of 5.9liters/hour for 1 hour to polymerize 2.8 g, per gram of the Ti catalystcomponent (A), of propylene. After the pre-polymerization, the liquidportion was removed by filtration, and the separated solid portion wasreslurried in decalin.

[Polymerization]

(1) A 50-liter autoclave was fully purged with propylene. Propylene(13.5 kg) and 250 Nl of hydrogen were added to the autoclave, and 7.2mmoles of triethylaluminum, 0.72 mmole of n-decyltriethoxysilane and0.09 mg-atom, calculated as Ti atom, of the Ti catalyst component werealso added at 65° C. Propylene was polymerized at 70° C. for 20 minutes.

(2) By releasing the pressure, propylene was removed, and thepolymerization was carried out for 1.5 hours at 75° C. while feedingpropylene so as to maintain the pressure of the inside of the autoclaveat 15 kg/ cm² _(G).

(3) One hour later, the pressure was released, and a gaseous mixture ofethylene and propylene (ethylene/ propylene mole ratio 50/50) was fed ata rate of 425 Nl/hr at 65° C. for 2 hours to give a block copolymer. Noadhesion to the wall of the autoclave occurred, nor was there anyformation of a lumpy product in the autoclave. The resulting powder hada high apparent density and good flowability. The amount yielded of theblock copolymer was 6.9 kg. The powder had an apparent density of 0.41g/ml, a melt flow index of 11.0 g/10 minutes, an [η] of 2.10 dl/g and anethylene content of 24 mole%. It contained 25% by weight of a componentsoluble in decane at room temperature.

An antioxidant was added to the block copolymer, and the mixture wasgranulated. Test pieces were prepared from the granules. The copolymerwas found to have a falling impact strength (-30° C.) of 270 kg/cm, anIzod impact strength (-30° C.) of 12.5 kg-cm/cm and a flexural modulusof 9500 kg/cm².

The polypropylene formed in the polymerization step (1) had an [η]_(FC)of 0.82 dl/g and a boiling n-heptane extraction residue of 97%. Thepolypropylene obtained in the polymerization step (2) had an [η]_(XC) of1.31 dl/g.

The [η]_(FC) /[η]_(XC) ratio was therefore 0.63. The amount ofpolymerization in (1) and (2) was 40% of that in the propylenepolymerization step.

EXAMPLE 3 [Preparation of a solid Ti catalyst component (A)]

Silicon dioxide (#952, a tradename for a product of Davison Company) wascalcined in a nitrogen stream at 200° C. for 2 hours and then at 700° C.for 5 hours. Ten grams of the calcined silicon dioxide, 40 ml ofpurified n-heptane and 40 ml of a 20% n-heptane solution ofn-butylethylmagnesium were put in a 500 ml flask, and reacted at 80° C.for 1 hour. Furthermore, 30 ml of purified n-heptane was added, and thereaction was carried out at 90° C. for 2 hours. After the reaction, thesupernatant was removed, and the residue was washed five times with 100ml of purified n-heptane. Finally, 40 ml of purified n-heptane was addedto the suspension to adjust its total amount to about 70 ml. Thesuspension was then cooled to 0° C., and a solution composed of 19.2 gof trichloroethanol and 20 ml of purified n-heptane was added dropwiseto the suspension at 0° C. over about 30 minutes. The mixture wasmaintained further at the above temperature for 1 hour, and then heatedto 80° C. over 1 hour. The reaction was carried out at this temperaturefor 1 hour. The supernatant was removed, and the residue was washed with100 ml of purified n-heptane twice and then with 100 ml of purifiedtoluene three times. Finally, purified toluene was added to adjust thetotal amount of the suspension to 200 ml. A 25 ml portion of thesuspension (200 ml) was uniformly taken into a 400 ml glass receptacle,and 55 ml of purified toluene was added. Then, 1.0 ml of di-n-butylphthalate was added, and the mixture was reacted at 50° C. for 2 hours.Then, 52.5 ml of TiCl₄ was added, and the reaction was carried out at90° C. for 2 hours. The liquid portion was removed by filtration, andthe solid portion was collected. The solid portion was washed with 100ml of purified n-heptane four times to give a solid catalyst component(A). The catalyst component (A) contained 3.3% by weight of titanium,4.2% by weight of magnesium, 17% by weight of chlorine and 5.2% byweight of di-n-butyl phthalate.

[Polymerization]

(1) A 50-liter autoclave was purged fully with propylene, and 13.5 kg ofpropylene, 26 mmoles of triethylaluminum, 4 mmoles ofn-propyltriethoxysilane and 0.62 g of the Ti catalyst component wereadded to the autoclave at 23° C. After adding 65 Nl of hydrogen,propylene was polymerized at 75° C. for 50 minutes.

(2) Then, propylene was removed by releasing the pressure, and thepolymerization was carried out for 1 hour at 80° C. while feedingpropylene so as to maintain the pressure of the inside of the autoclaveat 15 kg/ cm² G.

(3) One hour later, the pressure was released, and a gaseous mixture ofethylene and propylene (ethylene/ propylene mole ratio 60/40) was fed ata rate of 505 Nl/hr at 65° C. for 2 hours to form a block copolymer. Theamount yielded of the block copolymer was 5.7 kg. It had a melt flowindex of 38.5 g/10 minutes, an [η] of 1.71 dl/g and an ethylene contentof 13.5 mole%. It contained 15.1% by weight of a component soluble indecane at room temperature.

An antioxidant was added to the block copolymer, and the mixture wasgranulated. Test pieces were prepared from the granules. The blockcopolymer was found to have a falling dart impact strength (-30° C.) ofmore than 210 kg-cm, an Izod impact strength (0° C.) of 4.8 kg-cm/cm anda flexural modulus of 11900 kg/cm².

The polypropylene formed in the polymerization step (1) had a [η]_(FC)of 0.92 dl/g and a boiling n-heptane extraction residue of 96%. Thepolypropylene formed in the polymerization step (2) had a [η]_(XC) of1.21 dl/g.

The [η]_(FC) /[η]_(XC) of 0.76. The amount of polymerization in thefirst-stage polymerization steps (1) and (2) was 65% of that in thepropylene polymerization step.

What is claimed:
 1. A process for producing a propylene block copolymer having an MFR value of at least 10 g/10 minutes in the presence of a catalyst system formed from(A) a solid titanium catalyst component containing magnesium, titanium, halogen and a polycarboxylic acid ester as essential ingredients formed by contacting a magnesium compound, a titanium compound and the polycarboxylic acid ester, (B) an organoaluminum compound, and (C) an organic silicon compound represented by the following formula (I)

    SiR.sup.11.sub.m (OR.sup.12).sub.4-m                       (I)

wherein R¹¹ represents an alkyl or alkenyl or alkenyl group, R¹² represents a hydrocarbon group, and m is 1≦m≦2,which comprises (I) polymerizing or copolymerizing propylene in a first polymerization stage consisting of at least two steps wherein a crystalline propylene polymer having an intrinsic viscosity [μ]_(FC), measured in decalin at 135° C., of less than 1.8 dl/g is produced in the first step, the polyermization or copolymerization is carried out further in the subsequent steps, and from the final step, a crystalline propylene polymer from the first stage is obtained having an intrinsic viscosity [μ]_(XC), measured in decalin at 135° C., of 0.7 to 2.0 dl/g, the [μ]_(FC) /[μ]_(XC) ratio being adjusted to not more than 0.9, and (II) random-copolymerizing propylene and other alpha-olefin in a mole ratio of from 20/80 to 80/20 in a second polymerization stage in the presence of said crystalline polypropylene.
 2. The process set forth in claim 1 wherein in the first step of the first polymerization stage, a crystalline propylene polymer having an intrinsic viscosity [η]_(FC) of 1.5 to 0.4 dl/g is formed.
 3. The process set forth in claim 1 wherein in the first step of the first polymerization stage, a crystalline propylene polymer having an intrinsic viscosity [η]_(FC) of 1.0 to 0.45 dl/g is formed.
 4. The process set forth in claim 1 wherein a crystalline propylene polymer having an [η]_(XC) of 1.0 to 1.5 dl/g is obtained from the final step of the first polymerization stage.
 5. The process set forth in claim 1 wherein the ratio of the [η]_(FC) of the crystalline propylene polymer formed in the first step of the first polymerization stage to the [η]_(XC) of the crystalline propylene polymer obtained in the final step of the first polymerization stage, [η]_(FC) /[η]_(XC), is not more than 0.8.
 6. The process set forth in claim 1 wherein the ratio of the [η]_(FC) of the crystalline propylene polymer formed in the first step of the first polymerization stage to the [η]_(XC) of the crystalline propylene polymer obtained in the final step of the first polymerization stage, [η]_(FC) /[η]_(XC), is from 0.78 to 0.5.
 7. The process set forth in claim 1 wherein the proportion of the crystalline propylene polymer formed in the first step of the first polymerization stage is 10 to 95% by weight based on the entire crystalline propylene polymer formed in the first polymerization stage.
 8. The process set forth in claim 1 wherein the proportion of the crystalline propylene polymer formed in the first step of the first polymerization stage is 2 to 90% based on the propylene copolymer obtained from the second polymerization stage.
 9. The process set forth in claim 1 wherein the proportion of the crystalline propylene polymer formed in the first polymerization stage is 50 to 95% by weight based on the propylene copolymer obtained from the second polymerization stage. 